S-adenosylhomocysteine hydrolase plays a critical role in regulating AdoMet-dependent methylations in eukaryotic cells by regulating the ratio of AdoMet/AdoHcy. Several approaches are being used to determine the structure and function of this enzyme. 1) Structure Determination: The enzyme has been purified from rat liver, and cloned from a rat liver cDNA library. The amino acid sequence was determined and a putative NAD binding site identified. The rat liver enzyme has been expressed in E. coli and site-directed mutagenesis is in progress to determine the function of specific amino acid residues. Conformational changes for active and inactive forms of the enzyme have been examined by fluorescence, and circular dichroism. 2) Ligand Binding and Kinetic Properties: The role of NAD, nucleotide, and cAMP binding in regulating the catalytic activity has been studied, and photoaffinity ligands are being used to label the binding sites. A large number of adenosine and adenosylhomocysteine analogs have been examined for their ability to function as inhibitors and/or substrates of the enzyme. 3) Biological Effects of Inhibitors: In vivo these adenosine analogs can form very potent and specific inhibitors of transmethylation reactions, and these inhibitors have a wide range of biological activities, including antiviral activity against several RNA and DNA viruses, inhibition of leukocyte chemotaxis, and stimulation of cell differentiation.